1. Field of the Invention
The present invention relates to a radio communication apparatus or a mobile station that may be used, for example, in a mobile communication system introducing the W-CDMA (UMTS) communication system.
2. Description of the Related Art
At present, the standardization of the W-CDMA (UMTS) system, one of the third generation mobile communication system, is continued by the 3GPP (3rd Generation Partnership Project). One of the aspects of the standardization is the specification of HSDPA (High Speed Downlink Packet Access) to realize the maximum transmission rate of about 14 Mbps in the downlink.
The HSDPA employs the Adaptive Modulation and Coding (AMC), which, for example, may be characterized by adequately switching the QPSK modulation scheme and the 16 QAM scheme in accordance with the radio environment between the base station and mobile station.
Moreover, for the HSDPA, the H-ARQ (Hybrid Automatic Repeat request) is employed. In the H-ARQ when an error is detected in the received data from a base station, a mobile station sends a re-transmission request to the base station. The base station having received this re-transmission request retransmits the data. Accordingly, the mobile station executes the error correction decoding using both the data already received and the retransmitted received data. If an error is detected in the H-ARQ, the number of times of retransmission is reduced by effectively using the data already received in the H-ARQ system.
The radio channels used for the HSDPA include the HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel) and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
Both HS-SCCH and HS-PDSCH are shared channels of the downlink (namely, in the direction to a mobile station from a base station) and the HS-SCCH is the control channel for transmitting various parameters of the data transmitted with the HS-PDSCH. Various parameters may include modulation type (scheme) information indicating the modulation scheme to be used, the number of spreading codes (number of codes) to be assigned, and the information of the pattern of rate matching process to be executed before transmission.
Meanwhile, the HS-DPCCH is the individual control channel of the uplink (namely, direction to a base station from a mobile station). For example, this control channel is used by the mobile station to transmit, to the base station, the acknowledgment or no-acknowledgement of reception of the data received from the base station via the HS-PDSCH as the ACK signal or NACK signal. When the mobile station fails reception of data (when a CRC error is detected in the received data or the like), since the mobile station transmits the NACK signal as the retransmission request, the base station executes the retransmission control. If neither the ACK signal nor the NACK signal is received (in the case of DTX), the radio base station executes also the retransmission control. Accordingly, the DTX condition which occurs when the mobile station transmits neither the ACK signal nor the NACK signal may be considered as one of the retransmission request.
Moreover, the HS-DPCCH is also used to transmit the reception quality information of the received signal from the base station measured by the mobile station (for example, SIR) to the base station as the CQI (Channel Quality Indicator) information. The base station changes the transmission format of the downlink with the CQI information received. Namely, when the CQI information indicates that the radio environment in the downlink is good, the transmission format is switched to the modulation scheme which enables data transmission in the higher rate. When the inverse CQI information indicates that the radio environment in the downlink is bad, the transmission format is switched to the modulation scheme enabling data transmission in the lower transmission rate (namely, adaptive modulation).
[Channel Format]
Next, a channel format of the HSDPA will be described.
FIG. 1 illustrates a channel format of the HSDPA. Since the W-CDMA employs a code division multiplex method, each channel is demultiplexed by the code.
First, the channels not described above will be explained. The CPICH (Common Pilot Channel) and the P-CCPCH (Primary Common Control Physical Channel) are respectively the common channels of the downlink.
The CPICH is the channel used in the mobile station for channel estimation, cell search and as the standard timing of the physical channel of the downlink in the same cell. This channel is so-called used for transmission of the pilot signal. The P-CCPCH is the channel provided for each cell and used to transmit the broadcast information.
Next, the timings of channels will be explained with reference to FIG. 1.
As illustrated in FIG. 1, each channel forms one frame (10 ms) with 15 slots (each slot corresponds to the length of 2560 chips). As described above, since the CPICH is used as the reference channel of the other channels, the leading frames of the P-CCPCH and HS-SCCH frames are matched with the leading frame of the CPICH frames. Here, the leading frame of the HS-PDSCH frames is delayed by 2 slots for the HS-SCCH or the like. Accordingly, the mobile station is capable of demodulating the HS-PDSCH with the demodulation scheme corresponding to the received modulation scheme after reception of the modulation scheme information via the HS-SCCH. Moreover, the HS-SCCH and HS-PDSCH form one subframe with three slots.
The HS-DPCCH is the uplink channel and the first slot thereof is used to transmit the ACK/NACK signal as the response signal for verifying reception to a base station from a mobile station after passage of about 7.5 slots from reception of the HS-PDSCH. Moreover, the second and third slots are used for periodical feedback transmission of the CQI information for the adaptive modulation control to the base station. Here, the CQI information to be transmitted is calculated on the basis of the receiving environment (for example, result of SIR measurement of CPICH) measured during the period up to before one slot from before four slots of the CQI transmission.
FIG. 2 illustrates a CQI table for using the SIR (Signal to Interference Ratio) of the CPICH.
As illustrated in the figure, the table defines, for respective CQI information pieces 1 to 30, the correspondence relationship among the number of TBS (Transport Block Size) bits, number of codes, modulation scheme, and CPICH-SIR.
Here, the number of TBS bits indicates the number of bits to be transmitted in one subframe, the number of codes indicates the number of spreading codes used for transmission of the HS-PDSCH, the modulation scheme indicates any of QPSK and QAM used.
As is apparent from the figure, the better the SIR of CPICH is (SIR is larger), the larger the CQI becomes. When the CQI is larger, the number of corresponding TBS bits and the spreading codes becomes larger and the modulation scheme is switched to the QAM modulation. Accordingly, when the SIR is better, the transmission rate is faster, and the more radio resources (spreading code or the like) are used.
For example, the table illustrated in the figure may be stored in the memory of the mobile station. As explained previously, the mobile station periodically measures the SIR of the CPICH in the receiving environment measuring period and transmits CQI corresponding to the SIR measured with reference to the stored table to the base station by identifying the same CQI.
The base station achieves the transmission control considering the receiving environment of the CPICH in the mobile station by executing the adaptive modulation control described previously in accordance with the CQI information received. The channel format of the HSDPA has been briefly explained above.
Contents of the HSDPA described above is disclosed in the 3G TS 25. 212 (3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD) V6. 2.0 (June 2004).
According to the related art described previously, a mobile station can smoothly realize transmission control with respect to the radio environment between the mobile station and a base station by periodically measuring the reception quality of the downlink signal and by periodically transmitting the parameter information (CQI information) used for the adaptive modulation control to the radio base station.
However, when the transmission period (cycle) of the CQI information is longer and when the same CQI information is repeatedly transmitted several times corresponding to a single trial of measurement, mismatching may easily be generated between the radio environment between the base station and mobile station to actually conduct the adaptive modulation control and the CQI information used for the adaptive modulation control.